Homologous recombination during meiosis is essential for proper chromosome segregation and thus for formation of euploid gametes. Meiotic recombination is initiated by double-strand breaks (DSBs) made by the SPO11 protein. This proposal addresses molecular mechanisms underlying mammalian DSB formation and recombination, using mouse as a model system. Aims are: 1. To define mechanisms that ensure X-Y recombination. Sex chromosomes pose significant challenges to male meiotic cells because the X and Y share only a small region of homology (the pseudoautosomal region, or PAR) within which DSB formation and recombination must occur in every meiosis. Studies supported by this grant uncovered unique structural properties and dynamic behaviors of the mouse PAR that appear critical for proper X-Y segregation. To define the molecular basis of these properties, the higher order structure of the PAR will be examined in female meiosis and in meiosis of males bearing a much longer PAR. These studies will determine what properties are intrinsic to the PAR. The genetic control of PAR recombination will also be examined, focusing on roles of SPO11 isoforms and the DNA damage response kinase ATM. 2. To elucidate mechanisms of meiotic recombination and the factors involved. Recombination leads to formation of both crossovers and noncrossovers; much is known about the pathways leading to these products in yeasts, but comparatively little is known about these mechanisms in mammals. These issues will be addressed using a novel assay that allows all chromatids involved in a single meiotic recombination event to be characterized. Using this mouse tetrad assay, recombination patterns in wild-type spermatocytes will be delineated. Recombination in the absence of ATM function will also be examined. 3. To determine how ATM controls the number and distribution of DSBs. ATM homeostatically controls DSB numbers via a negative feedback loop, but the molecular basis of this regulation is unclear. To address this issue, the mechanism by which ATM is activated in response to SPO11- generated DSBs will be determined, focusing on the role of the MRE11-RAD51-NBS1 complex. In addition, a new method for genome-wide DSB mapping will be applied to Atm-/- spermatocytes to provide insight into the influence of ATM on the distribution of DSBs across the genome.